(i) Technical Field
The present invention relates to an image processing apparatus, an image processing method, and a non-transitory computer readable medium.
(ii) Related Art
In general, in order to form a color image, separate plates that correspond to respective color components, such as cyan (C), magenta (M), yellow (Y), and, in some cases, black (K), are prepared, and screen processing is performed on images of the respective plates. An image of the original plate is multi-level, while an image of each plate on which screen processing has been performed is binary. For example, in the case of printing, a color image is printed by depositing ink using an ink corresponding to the plate if the binary value is either of the two values.
A technique has been available in which an image, each plate of which is binary as a result of screen processing, is converted into an image, each plate of which is multi-level.
In the case of processing a series of images, it is presumed that the screen rulings and angles of inputted images on which screen processing has been performed are not changed. In such a case, processing of detecting the screen ruling and angle of each image is wasteful, and it is sufficient to detect the screen ruling and angle from any one image and use the detection result for other images. However, an image used to detect the screen ruling and angle does not necessarily have plates corresponding to all color components that are used. For a plate corresponding to a color component not used in an image that is a detection target, the screen ruling and angle are not detected, and multi-level conversion is not performed if the color component is used in other images. If an image that is a detection target is a character image or an image in which a halftone is not used, that is, an image which includes only a solid area, the screen ruling and angle are not detected or erroneously detected, and erroneous multi-level conversion processing may be performed on a series of images.